The problem with the website has always been that it’s trying to accommodate too many audiences and failing to accommodate any one of them particularly well. Only by very carefully deciding who the website is for will one be able to motivate a better design.
Typical audiences include people interested in the project, people interested in development, people interested in using Stan, and experienced Stan users looking for general documentation or additional pedagogical material. The targeting is complicated by the fact the Stan ecosystem covers multiple components – an autodiff library, algorithm library, probabilistic programming language, and interfaces – which each potential audiences will interact with differently. Further complicating matters is the fact that using Stan well requires a reasonable statistical background and consequently the best way to guide new users will vary depending on their particular statistical background. Every introduction to Stan needs to be a little different, and a website trying to cater to new people will never be able to capture all of those introductions.
The original version of the current design tried to compromise with a landing page that directed a few of the audiences (namely potential devs and users) to the best place for them to start. People then added more directions to accommodate more audiences and quickly the whole thing became overburdened and confusing (the same thing is happening to the manual, but that’s another story).
At some point @ariddell and I discussed an iterative navigation scheme that would guide visitors to targeted starting pages, but the design quickly got out of hand before anything came out of it.
Anyways, site design is hard but it’s exceptionally hard for Stan given the breadth of the project and the diversity of the community. Before any design is considered I strongly recommend identifying a few audiences to prioritize and where they need to start.